In vitro expression of the intron-containing gene for T4 phage thymidylate synthase

Better late than early: delayed translation of intron-encoded endonuclease I-TevI is required for efficient splicing of its host group I intron

The td group I intron interrupting the thymidylate synthase (TS) gene of phage T4 is a mobile intron that encodes the homing endonuclease I-TevI. Efficient RNA splicing of the intron is required to restore function of the TS gene, while expression of I-TevI from within the intron is required to initiate intron mobility. Three distinct layers of regulation temporally limit I-TevI expression to late in the T4 infective cycle, yet the biological rationale for stringent regulation has not been tested. Here, we deleted key control elements to deregulate I-TevI expression at early and middle times post T4 infection. Strikingly, we found that deregulation of I-TevI, or of a catalytically inactive variant, generated a thymidine-dependent phenotype that is caused by a reduction in td intron splicing. Prematurely terminating I-TevI translation restores td splicing, full-length TS synthesis, and rescues the thymidine-dependent phenotype. We suggest that stringent translational control of I-TevI evolved to prevent the ribosome from disrupting key structural elements of the td intron that are required for splicing and TS function at early and middle times post T4 infection. Analogous translational regulatory mechanisms in unrelated intron-open reading frame arrangements may also function to limit deleterious consequences on splicing and host gene function.

Intron-associated splicing reactions in bacteriophage T4

Group I introns are present in at least three bacteriophage T4 genes: td, nrdB and sunY. The transcription products of these three genes have similar intron consensus regions and secondary structures, which render them capable of guanosine-mediated in vitro autocatalytic splicing reactions. Moreover, it has been shown that the 245-amino-acid protein encoded in the td intron expresses an endonuclease that cleaves near the joining site for the two exons in the intron-deleted thymidylate synthase gene. The intron-containing td gene is resistant to the enzyme. As in the case of other group I intron-containing genes that have been described in eukaryotes, which also encode site-specific endonucleases, the td intron is highly mobile and can insert into the intron-less td gene by a process initiated by endonuclease cleavage near the insertion site. Whether intron transposition reactions have any physiological significance to the phage, or represent an early imprint on the evolution of introns, remains to be determined.

A site-specific endonuclease and co-conversion of flanking exons associated with the mobile td intron of phage T4

The product of the td intron open reading frame (ORF) of phage T4 is required for high-frequency transfer of the intervening sequence from intron-plus (In+) to intron-minus (In-) alleles. In vivo studies have demonstrated that the td ORF product targets cleavage of td In- DNA, and that cleavage is correlated with intron inheritance [Quirk et al., Cell 56 (1989) 455-465]. In the present study we show by in vitro synthesis of the td intron ORF product, that the protein possesses endonuclease activity and efficiently cleaves double-stranded DNA at or near the site of intron integration. In addition, we demonstrate that intron insertion is accompanied by co-conversion of the flanking exon sequences. Co-conversion of markers within 50 nt surrounding the site of intron insertion occurred at a high frequency (80-100%), and decreased at greater distance from the intervening sequence. Co-conversion may provide a mechanism for maintaining exon-intron RNA contacts required for accurate splicing of the relocated intron. Cleavage of target DNA by an intron endonuclease and co-conversion of flanking exon sequences are both features associated with mobile introns of eukaryotes, indicating a common mechanism for intron transfer in the eukaryotic and prokaryotic kingdoms.

Activation of a cryptic 5' splice site in the upstream exon of the phage T4 td transcript: exon context, missplicing, and mRNA deletion in a fidelity mutant

A collection of 100 td mutants defective in phage T4 thymidylate synthase (TS) production was screened for splicing impairments. Splicing-defective mutants were identified by a rapid assay developed to detect imbalances in the td protein products (TS, the exon ligation product, and NH2TS, encoded by the pre-mRNA). Thirteen selected mutants, confirmed to be splicing defective by an RNA-oligodeoxynucleotide hybridization assay, were all shown to be inhibited in the first step of the group I splicing pathway, cleavage at the 5' splice site. Of these, only one, SC99, appeared to be a specificity mutant. Whereas the 12 other mutants had sequence changes within the functionally important 5' and 3' domains of the intron, SC99 was shown to be an exon mutant. The G----A change at residue -3 of the upstream exon of SC99 resulted in loss of normal 5' splice site recognition. Furthermore, activation of a remote cryptic splice site at residue -29 of the upstream exon and missplicing of mRNA that is deleted for 29 nucleotides of the 5' exon are characteristic for this mutant. These results underscore the role of exon sequences in guiding the fidelity of the splicing reaction and they raise provocative questions about the alignment of introns within exon contexts that are consistent with accurate splicing and synthesis of an intact gene product.

Interrupted thymidylate synthase gene of bacteriophages T2 and T6 and other potential self-splicing introns in the T-even bacteriophages

Southern hybridization analyses of procaryotic DNA from Escherichia coli, lambda bacteriophage, and T1 to T7 phages were carried out. The hybridization probes used consisted of DNA restriction fragments derived from the T4 phage intron-containing thymidylate synthase gene (td) and short synthetic oligodeoxynucleotides defining specific exon and intron regions of the gene. It was shown that intact as well as restricted DNA from the T-even phages hybridized not only to both T4 phage td intron- and exon-specific probes but also to probes defining the td 5' (exon I-intron) and 3' (intron-exon II) presplice junctions. These data strongly suggest that, analogous to the T4 phage, only the T2 and T6 phages among the procaryotes tested contain interrupted td genes. The td intervening sequence in each phage is roughly 1 kilobase pair (kb) in size and interrupts the td gene at a site analogous to that in the T4 phage. This was confirmed by data from Northern (RNA) hybridization analysis of td-specific in vitro transcripts of these phage DNAs. [alpha-32P]GTP in vitro labeling of total RNA from T4 phage-infected cells produced five species of labeled RNAs that were 1, 0.9, 0.83, 0.75, and 0.6 kb in size. Only the 1-, 0.9-, and 0.75-kb species were labeled in RNA from T2- or T6-infected cells. The commonly present 1-kb RNA is the excised td intron, which exists in both linear and circular forms in the respective T-even-phage-infected cells, while the 0.6-kb RNA unique to T4 may be the excised intron derived from the ribonucleotide reductase small subunit gene (nrdB) of the phage. The remaining labeled RNA species are likely candidates for other self-splicing introns.

Two domains for splicing in the intron of the phage T4 thymidylate synthase (td) gene established by nondirected mutagenesis

Of 97 nondirected T4 thymidylate synthase-defective (td) mutations, 27 were mapped to the intron of the split td gene. Clustering of these intron mutations defined two domains that are functional in splicing, each within approximately 220 residues of the respective splice sites. Two selected mutations, tdN57 and tdN47, fell within phylogenetically conserved pairings, with tdN57 disrupting the exon I-internal guide pairing (P1) in the 5' domain and tdN47 destabilizing the P9 helix in the 3' domain. A splicing assay with synthetic oligonucleotides complementary to RNA junction sequences revealed processing defects for T4tdN57 and T4tdN47, both of which are impaired in cleavage at the 5' and 3' splice sites. Thus prokaryotic genetics facilitates association of specific residue changes with their consequences to splicing.

Localization of the T4 phage ribonucleotide reductase B1 subunit gene and the nucleotide sequence of its upstream and 5' coding regions

The nucleotide (nt) sequence in a 757-bp [corrected] segment downstream from the intron-containing T4 phage thymidylate synthase gene (td) has been determined. This region was found to contain two open reading frames (ORFs). The first ORF(ORF2) [corrected] 261 bp [corrected] in length, is 24 [corrected] nt downstream from the td gene. The second ORF(ORF3) [corrected]) is 200 bp long at 558 [corrected] nt from the td gene and extends to the end of the Eco RI fragment. The amino acid (aa) sequence (66 aa residues) deduced from the second truncated ORF shows 59% homology to the sequence of the N-terminal portion of the ribonucleotide reductase large subunit of either Escherichia coli (B1 subunit) or mouse (M1 subunit). This tentatively identifies the truncated gene to be the 5' end of the T4 phage ribonucleotide reductase subunit B1 (nrdA) gene and pinpoints its exact location on the T4 phage genomic map. Southern hybridization analysis suggests good sequence homology among the nrdA genes of various T-even phages.

Multiple self-splicing introns in bacteriophage T4: evidence from autocatalytic GTP labeling of RNA in vitro

RNA from T4-infected cells yielded multiple end-labeled species when incubated with alpha-32P-GTP under self-splicing conditions. One of these corresponds to the previously identified intron from the td gene of T4, while others appear to represent additional group I introns in T4. Two loci distinct from the td gene were found to hybridize to a mixed alpha-32P-GTP-labeled T4 RNA probe. These mapped in or near the unlinked genes nrdB and nrdC. A fragment from the nrdB region that contains the intron has been cloned and shown to generate characteristic group I splice products with RNA synthesized in vivo and in vitro. Multiple introns, and the prospect that these occur within several genes in the same metabolic pathway, suggest a possible regulatory role for splicing in T4.

Characterization of the intron in the phage T4 thymidylate synthase gene and evidence for its self-excision from the primary transcript

The td gene contains a 735 bp open reading frame within its 1017 bp intron. A 12 nucleotide stretch may form a stable secondary structure with the putative Shine-Dalgarno sequence of the intron open reading frame and thus impair its translation. SP6 RNA polymerase transcripts of the td gene synthesized in vitro at 40 degrees C encompass a 2.7 kb primary transcript, a 1.7 kb mRNA, and a 1 kb intron RNA. The excised intron RNA consisted of linear and cyclized forms. RNAase H studies and resistance of the cyclized intron to linearization by HeLa cell debranching enzyme suggest it to be circular. Self-splicing of isolated td primary transcript occurred only marginally at 28 degrees C, but increased progressively to 50 degrees C, and required the presence of both Mg++ and a guanosine cofactor. An internal guide sequence is evident which may align the 5' splice site with the 3' end, presumably for precise exon ligation.

The intervening sequence RNA of Tetrahymena is an enzyme

A shortened form of the self-splicing ribosomal RNA (rRNA) intervening sequence of Tetrahymena thermophila acts as an enzyme in vitro. The enzyme catalyzes the cleavage and rejoining of oligonucleotide substrates in a sequence-dependent manner with Km = 42 microM and kcat = 2 min-1. The reaction mechanism resembles that of rRNA precursor self-splicing. With pentacytidylic acid as the substrate, successive cleavage and rejoining reactions lead to the synthesis of polycytidylic acid. Thus, the RNA molecule can act as an RNA polymerase, differing from the protein enzyme in that it uses an internal rather than an external template. At pH 9, the same RNA enzyme has activity as a sequence-specific ribonuclease.

RNA splicing and in vivo expression of the intron-containing td gene of bacteriophage T4

The splice junction sequence of td mRNA from T4-infected cells has been determined (5'....GGU-CUA....3') and shown to be identical to that of the RNA ligation product encoded by the cloned gene [Belfort et al. Cell 41 (1985) 375-382]. The RNA processing functions, T4 RNA ligase, T4 polynucleotide kinase, and the host prr gene product appear not to be essential for exon ligation; neither are the host endoribonucleases RNase III, RNase P and RNase E required for intron excision. While these results are consistent with the autocatalytic splicing mechanism demonstrated in vitro [Chu et al. J. Biol. Chem. 260 (1985) 10680-10688], they leave unanswered the question of which protein(s), if any, might stimulate the in vivo reaction. Analysis of the products of the cloned td gene has led to identification of two td-encoded polypeptides, namely a polypeptide corresponding to the exon-I-coding sequence (NH2-TS), and the catalytically active thymidylate synthase (TS). Kinetic and nucleotide sequence data provide evidence that NH2-TS is the product of the primary transcript and that TS is encoded by spliced mRNA. These results suggest that splicing may provide a switch controlling the relative expression of NH2-TS and TS, two proteins with markedly different temporal appearances despite their identical transcriptional and translational start sites.